1. Field of the Invention
This invention relates to new mold release agents for synthetic resins or plastics, preferably polycondensates and polyaddition compounds and especially polyurethanes.
2. Description of the Prior Art
The mold release agents according to the invention also act as lubricants in the production of plastics. Lubricants for synthetic resins generally bring about an improvement in the flow behavior of the plastics and prevent the plastics from sticking or adhering to parts of the processing and production machinery. Accordingly, they are crucially important in improving the molding properties and processing properties of plastics and represent an indispensable additive, particularly for the processing of thermoplastic polyurethanes or polyurethane ureas.
In almost every process, a mold release agent has to be used to prevent the moldings from adhering to the mold walls. In one method, the molds are carefully coated with a thin film of the release agent before the polyurethane composition is introduced. Waxes, soaps or oils are normally used. Although these so-called "external" release agents are effective, they have to be applied in a separate operation, during which the mold has to be taken out of production. Exact dosage of the release agent is often difficult because the release agent is applied by spray-coating or spread-coating and does not completely fill complicated molds, i.e. for example molds which have been finely engraved.
The difficulties mentioned above, which occur in particular in the case of polyurethanes (other plastics, for example polyolefins, present no mold release problems even without release agents), have resulted in the developement of "internal" release agents which consist mainly of fatty acid derivatives. The fatty acid derivatives are added to the polyurethane composition and provide for smooth removal of the part to be produced from the mold. However, the usefulness of internal release agents in automatic molding processes is limited because a buildup of the release agent occurs after several cycles so that the process has to be interrupted. "Internal" release agents of the type in question for polyurethanes are described, for example, in DE-A-2,307,589, DE-A-2,319,648.
Examples of state-of-the-art "internal" release agents include natural and synthetic fatty acid derivatives. Amides of C.sub.8 -C.sub.20 monocarboxylic acids (such as dodecylamide, decylamide, oleylamide or stearylamide) in quantities of about 0.3 to 5% are particularly suitable. Equally suitable are diamides of aliphatic monocarboxylic acids containing more than 9 carbon atoms and aromatic aliphatic diamines such as phenylene-bis-palmitylamide or ethylene-bis-stearylamide. Further examples include esters of fatty acids, preferably containing more than 10 carbon atoms, such as palmitic acid methylester or stearic acid butyl ester and also glycerides of carboxylic acids containing more than 8 carbon atoms. Polyethylene waxes and synthetic waxes, montan waxes and mixtures thereof may also be used. 0.3 to 5% by weight (based on polyurethane solids) of derivatives of fatty acids containing more than 12 carbon atoms, for example esters or amides of fatty acids are preferably used as lubricants. Metal soaps, for example, magnesium, calcium, zinc and aluminum stearates, are also used as lubricants. However, the disadvantage of the release agents mentioned above is due to their pronounced tendency to exude or effloresce.
The release agents naturally exert their releasing effect only on that surface of the product to which they migrate due to their incompatibility with the polyurethane. The releasing effect generally increases with the release agent content, relatively high concentrations leading to an undesirable gray bloom on the surface of the product which, although removable by wiping, increases in intensity or recurs in the event of prolonged storage. In polyurethane (polyurea) granulate stored for long periods before further processing, the concentration of release agent at the surface can become so great that the product cannot be uniformly taken in by an extruder. In addition, the concentration of release agent at the surface is always at its lowest just when an optimal releasing effect is required, i.e., in the freshly molded part. Thus, the effect of a large part of the release agent added is merely the unwanted optical surface phenomenom. In addition, in the case of composite materials, the exuded release agent can damage the other material. The object of the present invention is to provide improved and, above all, nonefflorescing mold release agents.